SteamOS is a fork (derivative) of Debian GNU/Linux. The first version (SteamOS 1.0) is called ‘alchemist’ and it is based on the Debian ‘wheezy’ (stable 7.1) distribution.

SteamOS is designed to run Steam and Steam games. It also provides a desktop mode which can run regular Linux applications. SteamOS currently provides a limited set of packages, but many Debian wheezy packages work fine on SteamOS, says Valve.

Steam OS can only run Linux games, so your Windows-only library is out. However, Valve is working on a system which will allow Windows games to be streamed from your Windows machine to the your Steam OS machine.

Valve’s probably covering its bases for future hardware support. If people want to use SteamOS but don’t know how to fiddle with bootloaders, UEFI can cover that pretty well. I imagine they also want to support Secure Boot in the future.